U.S. patent application number 10/402618 was filed with the patent office on 2003-10-30 for curvature-corrected band-gap voltage reference circuit.
Invention is credited to Coady, Edmond Patrick.
Application Number | 20030201821 10/402618 |
Document ID | / |
Family ID | 25403594 |
Filed Date | 2003-10-30 |
United States Patent
Application |
20030201821 |
Kind Code |
A1 |
Coady, Edmond Patrick |
October 30, 2003 |
Curvature-corrected band-gap voltage reference circuit
Abstract
This band-gap circuit overcomes the deficiencies of conventional
band-gap circuits by compensating for higher order temperature
effects, thereby increasing accuracy. A first resistor network
including two resistors is connect to a first transistor while a
second resistor network that includes one resistor is connected to
a second transistor. One resistor in the first resistor network has
a high temperature sensitivity, and therefore produces a
temperature dependent ratio of currents through the transistors.
The inverting input and noninverting input of an operational
amplifier are coupled to the collectors of the two transistors. The
emitter region of the second transistor is coupled to two
additional resistors which are connected in series to each other.
The emitter region of the first transistor is coupled to the
junction between these two additional resistors. The output of the
operational amplifier is coupled to the bases of the transistors.
Introducing a temperature dependent current ratio through the
transistors allows for correction of higher order temperature terms
previously ignored by prior art band-gap circuits.
Inventors: |
Coady, Edmond Patrick;
(Colorado Springs, CO) |
Correspondence
Address: |
BLAKELY SOKOLOFF TAYLOR & ZAFMAN
12400 WILSHIRE BOULEVARD, SEVENTH FLOOR
LOS ANGELES
CA
90025
US
|
Family ID: |
25403594 |
Appl. No.: |
10/402618 |
Filed: |
March 27, 2003 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10402618 |
Mar 27, 2003 |
|
|
|
09894850 |
Jun 28, 2001 |
|
|
|
6563370 |
|
|
|
|
Current U.S.
Class: |
327/539 |
Current CPC
Class: |
G05F 3/30 20130101 |
Class at
Publication: |
327/539 |
International
Class: |
G05F 001/10 |
Claims
What is claimed is:
1. A band-gap voltage reference circuit comprising: a pair of
transistors each having base, collector and emitter electrodes,
wherein said base electrodes are connected together; a first
resistor network connected to a collector electrode of a first
transistor, wherein said first resistor network includes a first
resistor that has a high temperature coefficient, and a second
resistor that has a low temperature coefficient; a second resistor
network connected to a collector electrode of a second transistor,
wherein said second resistor network has a low temperature
coefficient; an amplifier device having its output terminal coupled
to said pair of transistors to produce an output signal responsive
to the difference between the voltages across said pair of resistor
networks; a feedback circuit coupled to said amplifier device and
developing a feedback signal corresponding to said output signal; a
differential current density device for establishing different
current densities in the separate transistors of said pair of
transistors, wherein said current densities vary in response to the
ratio of the first and second resistor networks; and an output
device that provides an output voltage that is temperature
compensated to at least a second order with respect to
temperature.
2. Apparatus as in claim 1, wherein the second resistor of said
first resistor network is connected in series to the first resistor
of said first resistor network.
3. Apparatus as in claim 2, wherein a ratio of current densities
changes as a function of temperature.
4. Apparatus as in claim 3, further comprising a second resistor
network that contains a third resistor that has a low temperature
coefficient connected in series with said second transistor
collector.
5. Apparatus as in claim 4, wherein said band-gap circuit comprises
a third resistor network connected to the emitter electrodes of
said transistors.
6. Apparatus as in claim 5, wherein said third resistor network
comprises a fourth resistor connected in series with the emitter
electrode of said first transistor.
7. Apparatus as in claim 6, wherein said third resistor network
comprises a fifth resistor connected in series with the emitter
electrode of said second transistor.
8. Apparatus as in claim 7, wherein said first second fourth and
fifth resistors have a negligible temperature coefficient.
9. Apparatus as in claim 8, wherein said band-gap circuit output
voltage has a temperature coefficient of less than 0.2 parts per
million/degree Celsius.
10. A band-gap voltage reference circuit comprising: a first and
second transistor each having a base, collector and emitter
electrodes, wherein said base electrodes of said first and second
transistors are connected together; an amplifier device having its
inverting input coupled to the emitter of a first transistor and
its noninverting input coupled to the emitter of a second
transistor to produce an output responsive to the difference
between the currents through said first and second transistors; a
differential current density device for producing different current
densities in said first and second transistors, wherein a ratio of
the current densities is temperature dependent; and an output
terminal of said amplifier device, wherein said output terminal
develops a substantially temperature independent voltage.
11. Apparatus as in claim 10, further comprising a first resistor
network connected to a collector of the first transistor, wherein
said first resistor network comprises at least two resistors in
series with said collector, wherein a first resistor in said first
resistor network has a high temperature coefficient.
12. Apparatus as in claim 11, wherein said resistor network
comprises a resistor with a high temperature coefficient and a
resistor with a low temperature coefficient.
13. Apparatus as in claim 12, wherein a second resistor network is
connected to the collector of the second transistors.
14. Apparatus as in claim 13, wherein said second resistor network
comprises a resistor that has a low temperature coefficient.
15. Apparatus as in claim 14, wherein the output voltage of the
band-gap circuit has a temperature coefficient of less than 0.2
parts per million per degree Celsius.
16. A method of compensating for second order temperature
coefficients of band-gap output voltage reference circuits
comprising the steps of: providing a pair of transistors, wherein
each transistor has base, collector and emitter electrodes, wherein
the transistors include a first transistor and a second transistor;
providing an amplifier device having its noninverting input coupled
to one transistor and further having its inverting input coupled to
a second transistor, thereby producing an output responsive to the
difference between the currents through said pair of transistors;
producing different current densities in said pair of transistors,
wherein a ratio of the current densities is temperature dependent;
providing an output circuit for said amplifier device, wherein said
output circuit includes an output terminal for developing an output
voltage; providing a first resistor network comprising two
resistors connected in series to the collector of said first
transistor; providing a first resistor in said first resistor
network that has a high temperature coefficient so that the output
voltage of the band-gap circuit is temperature compensated to at
least a second order.
17. The method as in claim 16, further including the step of
providing a second resistor network connected to the emitters of
said transistors.
18. The method as in claim 17, wherein said first resistor network
contains a second resistor that has a low temperature
coefficient.
19. The method as in claim 18, further including the step of
providing all the resistors in the second resistor network to have
low temperature coefficients.
20. The method as in claim 19, wherein the temperature coefficient
of said band-gap circuit output voltage is less than 0.2 parts per
million per degree Celsius.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] Not applicable.
FIELD OF THE INVENTION
[0003] The instant invention relates to band-gap voltage reference
circuits, and specifically to the class of band-gap circuits which
provide a higher degree of temperature stability by correcting for
higher order linearity terms.
BACKGROUND OF INVENTION
[0004] Band-gap voltage reference circuits provide an output
voltage that remains substantially constant over a wide temperature
range. These reference circuits operate using the principle of
adding a first voltage with a positive temperature coefficient to a
second voltage with an equal but opposite negative temperature
coefficient. The positive temperature coefficient voltage is
extracted from a bipolar transistor in the form of the thermal
voltage, kT/q (V.sub.T), where k is Boltzman's constant, T is
absolute temperature in degrees Kelvin, and q is the charge of an
electron. The negative temperature coefficient voltage is extracted
from the base-emitter voltage (V.sub.BE) of a forward-biased
bipolar transistor. The band-gap voltage, which is insensitive to
changes in temperature, is realized by adding the positive and
negative temperature coefficient voltages in proper
proportions.
[0005] A conventional prior art band-gap circuit is shown in FIG.
1. In prior art circuits such as this, all the resistors are
manufactured similarly, so the ratio of R3 20 to R4 30 would remain
constant with respect to temperature. An operational amplifier 10
maintains an equal voltage across R3 20 and R4 30, thereby keeping
the ratios of currents (IC1 to IC2) into the collectors of Q1 40
and Q2 50 equal over temperature also. It can be seen that IC1 is
inversely proportional to R3 and current IC2 is inversely
proportional to R4 30. The emitter areas of transistors Q1 40 and
Q2 50 are in a ratio of A to nA with the emitter area of Q2 50
scaled larger than that of Q1 40 by a factor of n. The resulting
collector currents and base to emitter voltages of the two
transistors result in a voltage across R1 that equals kT/q
ln(n.times.IC1/IC2), where ln is the natural logarithm function and
n is the factor by which the emitter area of Q2 50 is scaled larger
than that of Q1 40. The voltage across R1 is amplified across R2 by
the factor of 2.times.R2/R1.
[0006] The band-gap circuit functions by taking output voltages
that are positively and negatively changing with respect to
temperature, and adding them to obtain a substantially constant
output voltage with respect to temperature. Specifically, the base
to emitter voltage, V.sub.BE of Q1 40 has a negative temperature
coefficient, while the voltage across R2 has a positive temperature
coefficient. By taking the output voltage of the circuit at the
base of Q1 40, the positive and negative temperature coefficients
essentially cancel, so the output voltage remains constant with
respect to temperature.
[0007] A first-order analysis of a band-gap reference circuit
approximates the positive and negative temperature coefficient
voltages to be exact linear functions of temperature. The positive
temperature coefficient voltage generated from V.sub.T is in fact
substantially linear with respect to temperature. The generated
negative temperature coefficient voltage from the V.sub.BE of a
bipolar transistor contains higher order non-linear terms that have
been found to be approximated by the function Tln(T), where ln(T)
is the natural logarithm function of absolute temperature. When the
band-gap voltage is generated using conventional circuit
techniques, the Tln(T) term remains and is considered an error term
which compromises the accuracy of the reference output voltage.
[0008] What is needed is a more accurate band-gap reference circuit
that corrects for errors resulting from temperature changes that
lead to errors in the reference voltage.
SUMMARY OF INVENTION
[0009] The present invention solves the above-referenced problems.
It is an object of the present invention to improve the accuracy of
band-gap voltage reference circuits with variations in ambient
temperature. Conventional band-gap circuits exhibit a variation in
output voltage when ambient temperature changes. Conventional
band-gap output voltages will exhibit a parabolic characteristic
when plotted versus temperature on a graph. The present invention
reduces the magnitude of this voltage error by adding an equal but
opposite parabolic term to the voltage reference to cancel the
second order temperature drift term inherently found in
conventional band-gap circuitry.
[0010] In accordance with the present invention, a resistor that
has a high temperature coefficient is added to the collector of a
transistor.
BRIEF DESCRIPTION OF DRAWINGS
[0011] These and other objects, features, and characteristics of
the present invention will become apparent to one skilled in the
art from a close study of the following detailed description in
conjunction with the accompanying drawings and appended claims, all
of which form a part of this application. In the drawings:
[0012] FIG. 1 shows a conventional PRIOR ART band-gap circuit.
[0013] FIG. 2 shows the band-gap circuit of the instant
invention.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY
EMBODIMENTS
[0014] The band-gap reference circuit of the present invention, as
described with reference to FIG. 2, compensates for the Tln(T)
variation found in conventional implementations of band-gap
circuits. In the following description, various aspects of the
present invention will be depicted. However, it will be apparent to
those skilled in the art that the present invention may be
practiced with only some or all aspects of the present invention.
For purposes of explanation, specific configurations are set forth
in order to provide a thorough understanding of the present
invention. However, it will also be apparent to one skilled in the
art that the present invention may be practiced without the
specific details. In other instances, well known features are
omitted or simplified such that the present invention is not
unnecessarily obscured.
[0015] This invention comprises a source voltage VCC, resistors R1
120, R2 130, R3 140, R4 150, and R5 160, transistors Q1 170 and Q2
180 and one operational amplifier A1 190. A prior art band-gap
reference circuit with no compensation for Tln(T) will be referred
to with reference to FIG. 1. In accordance with the present
invention as described in FIG. 2, resistors R4 150 and R5 160 form
a first resistor network (RNET 1) that is connected in series and
provide a current IC2 into the collector of Q2 180. Similarly,
resistor R3 140 may be considered as a second resistor network that
is connected in series with the collector of Q1 170 and will draw a
current IC1 from VCC into the collector of Q1 170. Various circuit
techniques may be used to equalize the voltage across the first and
second resistor networks. One such technique is to connect the
non-inverting and inverting inputs of operational amplifier A1 190
to node 1 shown at 200 and node 2 shown at 210, respectively, and
to connect the output of the operational amplifier to the bases
230, 240 respectively of Q1 at 170 and Q2 at 180. The ratio of the
collector current of Q1 170 to the collector current of Q2 180 is
determined solely by the ratio of the resistance value of first
resistor network (RNET 1) to the second resistor network RNET2.
[0016] Prior art band-gap circuits have maintained a specifically
constant ratio between the collector currents of Q1 and Q2.
Referring back to FIG. 1, the prior art circuit uses identical
geometry resistors manufactured using the same process step to
maintain a constant ratio of R3 20 to R4 30 with variations in
temperature. It is known when a constant current-density ratio
greater than unity is maintained between Q1 40 and Q2 50 that a
voltage proportional to absolute temperature voltage is developed
between the emitters of Q1 40 and Q2 50. The current density ratio
of Q1 40 to Q2 50 is determined by resistor values R3 20 and R4 30
and emitter area ratio of Q2 50 to Q1 40, denoted as n in FIG. 1. 1
V R1 = kT q ln ( n R4 R3 ) ( 1 )
[0017] Equation (1), where k is Boltzmann's constant, q is the
charge of an electron, T is absolute temperature in Kelvin, and R3
20, R4 30 and n are as denoted in FIG. 1, shows that a voltage
proportional to temperature voltage is developed across R1 80. The
voltage across R1 80 is amplified by (1+R4/R3).times.(R2/R1) and
added to the base-emitter voltage of Q1 40 to create the band-gap
voltage.
[0018] Referring back to FIG. 2, the present invention purposely
introduces temperature dependence to the ratio of resistor networks
RNET1 and RNET2. This is a substantial departure from the
architecture of prior art band-gap circuits. Resistor R3 140 and R4
150 are preferably thin film resistors with a low temperature
coefficient of resistance (TCR). Resistor R5 160 is built in such a
way as to have a high TCR comparatively to R3 140 and R4 150. In
practice, various materials, such as a diffused resistor, can be
used to build R5 160 to realize a high value of TCR. 2 V R1 = kT q
ln [ n ( R 4 + R 5 ( T - T 0 ) TC R5 R 3 ) ] ( 2 )
[0019] From equation (2), it is apparent that the circuit
arrangement in the present invention introduces an additional term
that is equal to aTln(b+T), where a and b are constant terms
determined by the values R3 140, R4 150 and R5 160, the temperature
coefficient of R5 160 and the emitter area ratio of transistor Q2
180 to transistor Q1 170, denoted n. .DELTA.VR1 is then amplified
by (1+RNET1/RNET2).times.(R2/R1). By proper selection of these
circuit component values, the term aTln(b+T), can be set to
approximate the Tln(T) term that is arises in the base-emitter
voltage expression of Q1 170. With the addition of the Tln(T) term,
the output voltage at operational amplifier 190 is substantially
constant with respect to variations in temperature. The output of
the amplifier 190 is coupled in a feedback loop to develop a
feedback signal corresponding to the output signal. Therefore,
although circuit analysis is much more difficult with the
introduction of a temperature dependent current ratio into the pair
of transistors, this allows for correction of higher order terms
previously ignored in prior art band-gap circuits. It is noted that
disclosed is merely one method of creating a temperature dependent
current ratio, those skilled in the art may be able to produce
other such means to accomplish this. For example only one
particular method is disclosed for producing a temperature
dependent current ratio through the transistors. This temperature
dependent ratio may also be produced by introducing any type of
temperature variations between the first and second resistor
networks. If the first resistor network has a high temperature
dependence the second resistor network may have a substantial
temperature dependence also but different in magnitude from the
first resistor networks.
[0020] As the present invention may be embodied in several forms
without departing from the spirit or essential characteristics
thereof, it should also be understood that the above-described
embodiments are not limited by any of the details of the foregoing
description, unless otherwise specified, but rather should be
construed broadly within its spirit and scope as defined in the
appended claims, and therefore all changes and modifications that
fall within the metes and bounds of the claims, or equivalence of
such metes and bounds, are therefore intended to be embraced by the
appended claims.
* * * * *